In proportion as large scale integrated circuits (LSI) are heightened in integration level and operation speed, further fining the pattern rules is required. However, the light hitherto used in photoexposure technology contained different wavelengths, and these wavelengths were long. Consequently, there was a limit in fining the pattern rules. Thus, it was tried to use as light source g-line (436 nm) or i-line (365 nm) emitted from an ultrahigh-pressure mercury lamp. Even in those cases, however, the pattern rule limit was about 0.5 μm with respect to resolution, and the manufacture of LSI utilizing such photoexposure arts can attain at best the integration level corresponding to 16 Mbit DRAM.
In this context, deep ultraviolet (DUV) lithography using as a light source deep ultraviolet rays, which are shorter in wavelength than g-line and i-line, appears to offer promise as a new processing technology.
DUV lithography can achieve 0.1-0.3 μm resolutions in the imaging process, and can provide a pattern having effectively vertical walls with respect to the substrate if a resist having a low optical absorbance is used. Moreover, as this technology makes it possible to transfer a pattern in one operation, it offers a higher throughput than electron beam lithography.
In recent years, on the other hand, high intensity KrF excimer laser has been successfully used as the light source for DUV lithography. In order that DUV lithography using such a light source has practical utility in mass production of LSI, it is necessary to use resist materials having low optical absorbance and high sensitivity at the wavelength of that laser.
Thus, there have been lately developed the chemical amplification type resist materials which use an acid as a catalyst and possess not only sensitivities equivalent to or higher than those of conventional high sensitivity resists but also other excellent properties including high resolution and high dry-etching resistance (as proposed, e.g., by Liu et al in J. Vac. Sci. Technol., Vol. B6, p. 379 (1988)). As for the negative resists of the aforementioned type, Shipley Company is already marketing a three-component chemically amplified resist (trade name, SAL601ER7) which consists of a novolak resin, a melamine compound and an acid generator.
When negative resist materials are used in the manufacturing process of LSI, they can serve for wiring and gate forming processes in the LSI production, but it is difficult for them to form contact holes because fine processing techniques are required therein.
On the other hand, hitherto proposed chemically amplified positive resists have a defect such that when they are used without undergoing any modification in forming patterns in accordance with DUV, electron-beam or X-ray lithography the developed patterns tend to overhang in profile because of the lowering of the solubility at the resist surface (K. G. Chiong, et al., J. Vac. Sci. Technol., Vol. B7, (6), p. 1771, (1989)). This overhanging phenomenon is at disadvantage in making the dimensional control of the patterns difficult to result in impairing dimensional controllability in the processing of substrates by the use of a dry etching technique or, what is worse, in readily causing the collapse of the patterns.
Accordingly, there has been a strong demand for developing positive resist materials of chemical amplification type which are free from the above-described defect and have high performance.
In compliance with such a demand, Ito et al have proposed the chemically amplified positive resist material consisting of a resin called PBOCST, or a poly(hydroxystyrene) protected with t-butoxycarbonyl groups, and an onium salt (“Polymers in Electronics”, ACS Symposium Series, No. 242, American Chemical Society, Washington, D.C., 1984, p. 11).
However, the onium salt used therein contains antimony as a metal component, so that the substrate is contaminated with the antimony. In addition, the resist material recited above suffers from a very great change with the lapse of time after irradiation with DUV or the like.
Another positive resist material for DUV lithography has been proposed by Ueno et al, wherein poly(p-styreneoxytetrahydropyranyl) is used as the principal component and an acid generator is added thereto (36th Oyoo Butsuri Gakkai Kanren Rengo Koenkai, 1989, 1p-k-7).
The foregoing resist material, however, tends to undergo positive to negative inversion when exposed to deep ultraviolet rays, electron beams or X-rays.
Moreover, with the two-component positive resist materials as recited above, which are constituted of a resin, whose OH groups are protected with certain groups, and an acid generator, it is necessary to decompose many of the protected groups in order to render the resist soluble in a developer. The decomposition involves a considerably high risk of film thickness variations, in-film stress or air bubbles in the process of LSI production.
Such being the case, there have been developed three-component positive resist materials as chemically amplified positive resist systems which are free from defects of the foregoing two-component ones. The three-component resist system consists of an alkali-soluble resin, a dissolution inhibitor and an acid generator.
As a three-component positive resist material, the resist material RAY/PF (produced by Hoechst AG.), which contains a novolak resin, an acetal compound as a dissolution inhibitor and an acid generator, has been developed for X-ray lithography.
However, the resist sensitivity thereof closely depends on the time elapsed from the exposure to X-rays until the development, because the resist material RAY/PF undergoes chemical amplification at room temperature. Accordingly, it is necessary to systematically perform strict control of that time. In actual practice, however, it is not easy to strictly regulate the time between the exposure and developing steps. That material cannot therefore ensure dimensional stability to the patterns formed therein. In addition, it has another disadvantage in that its optical absorbance at the wavelength of KrF excimer laser beam (248 run) is so high that it is unsuitable for the lithography using that laser.
In general, in order to effect chemical amplification, many resist materials require a heat treatment after exposure (the so-called post-exposure baking, abbreviated as “PEB”). Although PEB is an additional processing step, compared with the case in which resist systems undergo chemical amplification at room temperature, it enables less severe regulation of the time between exposure and developing steps. Thus, the resist materials requiring PEB can bear stable resist characteristics.
In a resist system which undergoes hydrolysis in the chemical amplification step, water is required for the hydrolysis reaction, and the resist material must therefore contain an appropriate amount of water.
In many cases, organic solvents immiscible with water, such as ethoxyethyl acetate, are used as a solvent for coating a resist material on a substrate, and resins which themselves are not compatible with water are used as a constituent of resist materials. Under these circumstances, it is difficult to incorporate a predetermined amount of water in such resist materials, and even if water can be incorporated therein, it will be troublesome to control the water content.
On the other hand, the decomposition reaction of t-butoxycarbonyloxy group does not require any water. More specifically, two components alone, namely, t-butoxycarbonyloxy group and an acid as catalyst, take part in the progress of the reaction. Therefore, the decomposition reaction is more suitable for chemical amplification.
Moreover, many of the t-butoxycarbonyloxy containing compounds are known to inhibit the dissolution of novolak resins, which infers that t-butoxycarbonyloxy group has dissolution inhibiting effect on novolak resins.
Taking into account the knowledge described above, Schlegel et al have reported a three-component positive resist material consisting of a novolak resin, t-butoxycarbonyl protected bisphenol A as dissolution inhibitor and pyrogallol methanesulfonic acid ester (37th Oyoo Butsuri Gakkai Kanren Rengo Koenkai, Spring 1990, 28p-ZE-4).
Such a resist material is, however, difficult of practical use, because the novolak resin has high optical absorbance.
Schwalm et al have developed bis(p-t-butoxycarbonyloxyphenyl) iodonium hexafluoroantimonate as a compound in which two functions of the dissolution inhibitor and the acid generator are combined (Polymer for Microelectronics, Tokyo 1989, Session A38), and have reported the mixture of that compound with a novolak resin as a positive resist material for DUV lithography.
However, as the foregoing resist material contains not only the novolak resin having high optical absorbance but also the metal, it is not suitable for practical application.
On the other hand, it is known that in the chemically amplified positive resist materials of three-component type, which are constituted of a resin, a dissolution inhibitor and an acid generator, the acid generator used has a particularly great influence on the performance as a resist material.
Typical examples of such an acid generator include (C6H5)3S+−O3SCF3, (C6H5)3S+−PF6, (C6H5)3S+−SbF6, (C6H5SC6H4)(C6H5)2S+−O3CF3, CH3OC6H5(C6H5)2S+−OSO2CF3, and so on.
Of these acid generators, substituted or unsubstituted triphenylsulfonium compounds have a characteristic such that they are decomposed by irradiation with high energy beams, including ultraviolet rays and electron beams, to produce acids. Thus, these compounds have so far been used widely, e.g., as a photopolymerization initiator in cation polymerization, a photocuring agent for epoxy resins, an acid generator for photoresists, and so on. When hitherto used triphenylsulfonium compounds are incorporated as a constituent of resist materials, they can lower the solubility of the resists in aqueous alkali solutions and further can inhibit the resist film from thinning upon development, because they themselves are soluble in oils.
However, the dissolution speed of those resists in an aqueous alkali solution is lowered in the exposed area also, which corresponds to the space part of the resist pattern, since the decomposition products which the acid generators yield by absorbing high energy beams are also soluble in oils. In such resist materials, it is therefore impossible to enlarge a ratio of the alkali dissolution speed in the area irradiated with high energy beams to that in the area unirradiated therewith (this ratio is generally called “dissolution contrast”). Thus, when developed, the resist materials using the triphenylsulfonium compounds suffer from disadvantages of (1) not securing sufficient sensitivity, (2) providing low resolution, (3) being apt to forming a slightly soluble layer at the resist surface, (4) having insufficient etchability, and so on.
Meantime, acid generators other than the foregoing compounds have some of the disadvantages cited above, too. The resist materials containing them therefore suffer from the lowering of performance.